The present invention relates to a process and device for machining by electroerosion, with which an electrode-piece is machined by means of an electrode-tool separated from each other by a working space by applying electrical pulses between the electrode-tool and the electrode-piece by means of an electrical circuit comprising at least one low impedance voltage source and a regulation circuit.
Machining by electroerosion requires a generator of intermittent discharges, which is connected between the piece and the electrode, the latter having for its purpose to penetrate progressively into the piece thanks to the erosive action of the discharges.
As the electrode is subjected to only low wear, there is obtained a faithful hollow reproduction of the shape of the electrode in the case of a penetration along a single axis, thanks to a servomechanism which maintains an optimum distance between the piece and the electrode, a so-called sparking distance, and which is the order of several dozens of microns. Other servomechanisms can ensure the relative translatory and/or rotative movements along other axes and one takes account of the amplitudes of these movements to predetermine the deformation of the imprint. These movements and these servomechanisms will not be described nor even shown here, as they relate to well-known and mastered techniques. However, precise imprints cannot be obtained unless the initial dimensions of the electrode are maintained in the course of machining, and if this is not the case, it is necessary to provide several electrodes, which gives rise to supplemental costs and renders electroerosion less economical. The problem of providing a generator with very low wear is always present, although the performances in terms of volumetric wear are remarkable, because there are nowadays obtained, with current generators, certain favorable conditions, and values as low as 0.1%. Unfortunately, this wear is not equally distributed, it is in fact concentrated on the pointed portions and edges of the electrode, such that perfect reproduction of shape is not ensured.
The generators of the type with limitation of current by resistance are known from the time that it has been known to use transistors for generating intermittent discharges. FIG. 14 shows in a very simplified way the principle, particularly by symbolizing the transistor by a switch S. E0 is the internal voltage or vacuum voltage of the source, the current is limited by the resistance R, by the parasitic or distributed inductance of value L, and by the impedance of the space between the electrode and the piece. During an erosive discharge, this impedance is represented in an idealized way by an element which maintains the voltage Ug at its terminals, no matter what the current i. Of course, the physical reality is much more complicated, particularly by virtue of the fact that the electrode-piece voltage is substantially higher at the beginning of discharge, and develops toward a certain value Ug defined as being the mean statistic of the voltages at the end of discharge. For an understanding of what follows, it will be recalled that, at the beginning of a discharge, the current increases with a slope di/dt=(E0xe2x88x92Ug)/L and which tends asymptotically toward (E0xe2x88x92Ug)/R. This type of generator gives acceptable wear, but unfortunately obtaining very low wear requires, as will be seen later on, an adjustable voltage E0 which is very near Ug, such that the control of the asymptotic current requires again finely adjusting the value of R, which would be very difficult and costly.
The generators of the current source type form a second category which offers much flexibility as to the control of the current, because switching transistors require the current to follow a standard. They are thus capable of maintaining a practically constant current during discharge, no matter what the voltage at its terminals, or at least toward a certain limit.
But this type of generator has the drawback of producing current pulses with an initial slope that is much too steep, which is a cause of wear.
The object of the present invention is to obtain a process and a device which eliminate the mentioned drawbacks, and which permit reducing the wear of the electrode-tool to a minimum and to provide a perfect shape reproduction, whilst ensuring easy control of the current during and after the rise of the machining current.
The invention is characterized to this end by the fact that there is carried out an adjustment of the current rise of the electrical pulses as a function of the time after triggering a discharge between the electrode-tool and the electrode-piece such that a parameter connected to this current rise will be substantially equal to a reference size corresponding to a minimum wear value of the electrode-tool.
These characteristics permit obtaining a machining process and device ensuring very low wear of the electrode-tool. The initial dimensions and shape of this latter are maintained during the course of machining, which renders electroerosion very precise and economical. There is thus carried out upon triggering a discharge according to a first mode in which a transitory current arises under the influence of a low impedance voltage source, which source is connected between the electrode and the piece, the impedance being sufficiently low that the increase of the current will be determined not only by the electrical characteristics of the circuit, but also by the discharge voltage, the current rise having been adjusted to a minimum wear value predetermined by technological tests and, as soon as the transitory current reaches a desired value, one operates according to a second mode in which the current is essentially determined by the electrical characteristics of the circuit.
The principal advantage, which is the obtention of very low wear, has a great economical importance, because, as mentioned in the introduction, it permits not only machining with precision, but also reducing the number of electrodes required.
Another advantage is that the process can be introduced into various known types of generators: current limitation by resistance, current limitation by inductance and switching transistors arranged to create a current source and also a DC-DC converter. Moreover, the advantages inherent in the generator serving as a platform can be preserved. For example, a generator with a current source according to the known technique is transformed into a new generator permitting controlling the initial slope of the current increase of the discharges, which permits achieving adjustments ensuring very low wear. Moreover, the advantages of the discharge with a current source are maintained, particularly the absence of any additional element such as a resistance or inductance adapted to limit the current, this latter function being ensured by the rapid switchings of the power transistors. The flexibility of control of this generator permits it to generate a wide variety of forms of current pulses.
Preferably, the process is characterized by the fact that there is carried out the adjustment of the current rise of the electrical pulses during a first transitory phase and that the machining current is maintained during a second phase at at least a reference value as soon as the current increase has reached this reference value.
There is thus obtained a very low wear of the electro-tool associated with an effective electroerosion and easy control.
According to a preferred embodiment, the adjustment of the rise of current is carried out by means of a loop such that a statistical size representative of said parameter coincides with said predetermined reference size.
Given the fact that the adjustment of the rise of machining current is very critical, a mechanism permits ensuring reproducibility in the field of regimes with very low wear obtained in the laboratory, in spite of the disturbances such as differences in line lengths, aging and tolerance of the elements, variations of temperature, etc.
Very favorably, said adjustment is carried out by acting on the adjustable internal voltage of at least one low impedance adjustable voltage source.
These characteristics permit obtaining a very fine and precise adjustment of the slope of the increase of the machining current.
Preferably, the reference size corresponding to a minimum wear value is determined by causing the internal voltage of the adjustable voltage source to vary, and by observing the dispersion of the spectrum of the angles of the slope of increase of current of a given number of discharges.
The reference size could also be obtained when the dispersion of this spectrum is substantially maximum.
The surprising result of the measurements of wear demonstrate that a minimum wear is in direct correlation with the dispersion of the spectrum of the increases of the current of a series of discharges. There can thus be obtained, from the observation of the dispersion of the spectrum of the increases of current, the minimum wear conditions corresponding to a value of the reference size, such as the optimum slope of the increase of the machining current.
A preferred embodiment is characterized by the fact that there is carried out an automatic control of a mean slope of increase of current of the erosive discharges by means of successive servo groups, by starting a machining cycle with said internal voltage fixed to an initial value corresponding to a pre-established reference voltage, by spacing by a discrimination test of the contaminated discharges, short circuits and arcs, by determining the slope of increase and current of the retained discharges, by carrying out n=N cycles of machining by calculating the mean slope of increase of current over the N retained discharges, by comparing the mean slope of increase of current obtained at said reference slope, by decreasing respectively increasing the internal voltage by a predetermined adjustment value, if the mean calculated slope is greater than the reference slope plus an adjustment variation, respectively less than the reference slope decrease by this adjustment variation, so as to obtain a new initial value of said internal voltage and by carrying out successive servo loops with new initial values of the internal voltage.
This automatic control is easy to carry out with a simple control unit and permits reproducing over the field the best results as to wear.
In a surprising modification, said parameter is the size of the blocked discharges during which the low impedance voltage source does not decrease the current, the rise in current of the electrical pulses being adjusted such that the size of the blocked discharges coincides with a reference size corresponding to minimum wear.
These characteristics permit obtaining a process whose practice is easy to carry out and which ensures minimum wear results that are very favorable.
The process is thus favorably characterized by the fact that there is carried out an automatic control of the current rise of the erosive discharges by means of successive servo loops, by starting a machining cycle with said internal voltage fixed at an initial value corresponding to a pre-established reference voltage, by discarding by a discrimination test the contaminated discharges, by short circuits and arcs, by detecting the blocked discharges, by carrying out n=N machining cycles, by calculating the size of the blocked discharges, by comparing this latter to a pre-established reference size corresponding to a minimum wear of the electrode-tool, by limiting, respectively increasing the internal voltage by an adjustment value, if the calculated size is less than the reference size decreased by an adjustment variation, respectively greater than the reference size added to an adjustment variation, so as to obtain a new initial value of the internal voltage, and by carrying out successive servo loops with new initial values of the obtained internal voltage.
Of course the present invention also relates to a device for practicing a process of machining by electroerosion, comprising an electrode-tool separated by a working slot from an electrode-piece, an electrical circuit with at least one low impedance voltage source and a regulation circuit arranged so as to apply electrical impulses between the electrode-tool and the electrode-piece, characterized by the fact that it comprises adjustment means to adjust the current rise as a function of the time after triggering a discharge between the electrode-tool and the electrode-piece such that a parameter connected to this current increase will be substantially equal to a reference size corresponding to a minimum wear value of the electrode-tool.
This device permits obtaining very low wear of the electrode-tool and very precise machining. It also permits easy modification of the existing electroerosion generators for very low wear.
Preferably, the adjustment means are arranged so as to modify the adjustable internal voltage of at least one low impedance voltage source.
According to a preferred embodiment, the device is characterized by the fact that said parameter is the mean slope of the increase of machining current and by the fact that it comprises means adapted to regulate this mean slope such that it coincides with a pre-established reference slope for minimum wear of the electrode-tool.
These characteristics ensure easy and precise adjustment of the minimum wear conditions.
According to a preferred embodiment, the device is characterized by the fact that it comprises a first low impedance voltage source whose internal voltage can be adjusted by a control unit, a first branch connecting a first terminal of the first voltage source to the electrode-tool and comprising, in series, a second low impedance voltage source, a first switch, a first member for measuring a first current, a self-induction winding, a second switch, a second member for measuring a second current and a first diode, the input of the second switch being connected by a second diode to the first terminal of the first voltage source, and by the fact that the electrode-piece is connected to the second terminal of the first source by a third branch, itself connected by a third diode to the output of the first switch and by a fourth diode to the output of the second switch, the control unit receiving signals from the measuring members and addressing control signals to the first voltage source and to the switches.
There is thus obtained an electro-machining generator permitting obtaining very precise adjustment and servo control of the conditions of machining for minimum wear of the electrode-tool and an easy control of the machining current during all the cycle of one discharge.
Another very advantage embodiment is characterized by the fact that it comprises a first low impedance voltage source of which a first terminal is connected to the electrode-tool by a first branch comprising in series a first switch, an additional source of low impedance voltage whose internal voltage can be adjusted by a control unit, a measuring member of current and a first diode, by the fact that the electrode-piece is connected by a second branch with a second switch to the second terminal of the first voltage source, and by the fact that it comprises a third branch comprising a second diode connecting the inputs of the two switches and a fourth branch comprising a third diode connecting the outputs of the two switches, the control unit receiving signals from the measuring member and addressing control signals to the additional voltage source and to the switches.
This embodiment has a particularly simple and low cost construction permitting however easy control of the machining current in its transitory phase and in its regulated phase. Moreover, an adaptation of the invention to pre-existing circuits can be carried out.